Air Shower Array

When an ultra-high-energy cosmic-ray particle plunges
into the atmosphere of the Earth, it collides with an
atomic nucleus such as nitrogen and oxygen, and
produces multiple secondary particles. These secondary
particles in turn collide with atomic nuclei in the
atmosphere, and produce particles again. A number of
repetitions of this process form an extensive cascade
of secondary particles, which rains down on the ground
with a planar shape. This cascade of particles is
called an air shower. A primary gamma-ray particle
also induces an air shower through electromagnetic
reactions in the atmosphere. Using an array of
detectors on the ground to observe air-shower
particles, we can measure the energy and the arrival
direction of the primary cosmic-ray particle that
induced the air shower, based on the distribution of
particle densities and timings recorded by the
detectors. The experimental site is a plateau located
4,740 m above sea level halfway up Mt. Chacaltaya,
Bolivia. The altitude of this site corresponds to the
atmospheric depth where an air shower meets its great
development. We are planning to continuously observe
ultra-high-energy cosmic rays and gamma rays with the
ALPACA air shower array, currently designed as an
array of 401 surface particle detectors covering the
area of 83,000 m2.

Schematic view of the current design of the whole
ALPACA observatory. Small black squares, 401 in total,
are 1 m2 surface particle detectors that
compose the ALPACA air shower array.

Image of the ALPACA air shower array. The photo shows
the air shower array of the Tibet ASγ experiment,
located on a highland in Tibet at the altitude of
4,300 m.
Image Credit: Institute for Cosmic Ray Research, University of Tokyo